A technique for estimating road reaction torque, acting on a steering shaft, from motor voltage, motor current and steering torque, is proposed for conventional electric power steering systems (Patent Document 1, for instance).
Another electric power steering system is proposed which determines whether there is an increased steering angle and estimates, during the increased steering angle, rack thrust that is in a proportional relationship with the road reaction torque, from the motor current and the steering torque (Patent Document 2, for instance).